In the operation of industrial boiler systems, the common practice for many years has been to use a cation resin ion-exchanger, regenerated with sodium chloride, as a condensate filter or polisher. The purpose of such a filter or polisher has been twofold, namely to capture the particulate iron and copper ordinarily present in industrial plant systems and to remove any trace of calcium or magnesium hardness. In low-pressure boiler systems, the amounts of sodium ion released by the ion-exchange resin are generally of no consequence. However, in high-pressure boiler systems, i.e., those operating at about 1500 p.s.i. for example, polished condensate which contains the customary levels of sodium salts cannot be utilized with complete success as boiler feedwater. High-pressure boiler systems are particularly susceptible to internal corrosion, especially hydrate corrosion; and it is the ordinary practice in the operation of these systems both to use a volatile amine for inhibiting corrosion and to regulate the pH of the system with a phosphate, for example. It has been found that sodium ion, even in low concentration, is capable of releasing undesirable hydroxide ion in these systems thereby promoting corrosion. Moreover, when condensate containing a volatile amine corrosion inhibitor is regenerated by means of a sodium-cycle cation resin, cationic amines exchange for sodium ion, with the result that sodium ion is released to the system and the amine corrosion inhibitor is extracted.